Central California, already painfully stressed by the worst drought in 50 years (which the US Drought Monitor designates as “extreme or exceptional drought,” the most serious category on the agency’s five-level scale), has another problem with its water supply. Aquifers that supply drinking and irrigation water have recently had to swallow almost 3 billion gallons of tainted wastewater from nearby hydraulic fracturing.

Fracking involves blasting huge volumes—140,000 to 150,000 gallons of pressurized water, sand, and often unspecified “proprietary” chemicals per day—into layers of rock far underground. Oil and wastewater then come to the surface, and companies separate the two and reinject the water component into designated disposal wells. The problem occurs when they reinject it where it can enter aquifers containing pure water. Experts estimate that California has an estimated 2,583 wastewater injection wells throughout the state, 1,552 of them currently active.

The state’s Water Board confirmed beyond doubt that in Kern County, several oil companies used at least nine of 11 injection wells to dispose of waste contaminated with fracking fluids and other pollutants, including high levels of arsenic, thallium, and nitrates, into high-quality water protected under both federal and more tolerant state laws. Thallium is a component of rat poison. Arsenic, toxic in itself, can also compromise the immune system and cause cancer. The state’s Division of Oil, Gas and Geothermal Resources shut down the 11 Kern County oil field injection wells and began scrutinizing almost 100 others in July for posing a “danger to life, health, property, and natural resources.”

The Center for Biological Diversity, a national, nonprofit conservation organization, obtained documents relating to the illegal dumping. The state Water Board had written the US Environmental Protection Agency that the Central Valley Regional Water Board had discovered the violations. The state also said that 19 more injection wells may have also contaminated sensitive, protected aquifers. More than 100 water wells in northeast and east Bakersfield may also have been contaminated.

The Central Valley board has only been able to test eight water wells since June, but four turned out to contain toxic chemicals in illegal amounts. Experts estimate that California has an estimated 2,583 wastewater injection wells throughout the state, 1,552 of them currently active.

Palla Farms LLC, a 92-year-old farming company in Kern County, sued four oil producers in late September for neglect, failure to provide fluid treatment plans, and contaminating groundwater the farms use for irrigating cherry and almond trees.

Says Hollin Kretzmann, an attorney for the Center for Biological Diversity:

Clean water is one of California’s most crucial resources, and these documents make it clear that state regulators have utterly failed to protect our water from oil industry pollution. Much more testing is needed to gauge the full extent of water pollution and the threat to public health. But Governor Brown should move quickly to halt fracking to ward off a surge in oil industry wastewater that California simply isn’t prepared to dispose of safely.

Environmental activists in the Golden State are calling on Governor Jerry Brown to put a stop to water-intensive fracking for its role in aggravating the drought. (See this interactive map from the Washington Post for a look at the California water problem.)

Concerns other than contamination and the drought threat to California’s precious water supply include permanent changes to the quality of life throughout California and the oil industry’s contribution to climate change.